How many litres of a 90% of concentrated acid needs to be mixed with a 75%…
2025
How many litres of a 90% of concentrated acid needs to be mixed with a 75% solution of concentrated acid to get a 30 litre solution of 78% concentrated acid?
- A.
8
- B.
9
- C.
7
- D.
6
Attempted by 31 students.
Show answer & explanation
Correct answer: D
Let x be the litres of the 90% acid to use. Then (30 - x) litres of the 75% acid are used.
Set up the acid-content equation: 90% of x plus 75% of (30 - x) must equal 78% of 30.
Write the equation: 0.9x + 0.75(30 - x) = 0.78 × 30
Simplify the left side: 0.9x + 22.5 - 0.75x = 23.4, so 0.15x = 0.9.
Solve for x: x = 0.9 ÷ 0.15 = 6 litres.
Check: 0.90×6 + 0.75×24 = 5.4 + 18 = 23.4 litres of pure acid, and 23.4 ÷ 30 = 0.78 = 78%.
Answer: 6 litres of the 90% solution are needed (the remaining 24 litres should be the 75% solution).